U of T researchers explain the significance of the universe's recent 'baby pictures'

The Atacama Cosmology Telescope (ACT) collaboration, which includes researchers from the University of Toronto, recently produced the clearest images yet of the universe’s infancy from the earliest cosmic time accessible to humans.

Measuring light that has travelled for almost 14 billion years to reach a telescope high in the Chilean Andes, the two new images reveal the universe when it was about 380,000 years old – the equivalent of hours-old baby pictures of a middle-aged adult.

“We have produced two images of the very early universe from a time long before there were any stars and galaxies – when all of space was filled with an almost perfectly uniform mixture of hydrogen and helium gas, radiation and dark matter,” says Adam Hincks, an assistant professor in U of T’s David A. Dunlap department of astronomy and astrophysics in the Faculty of Arts & Science and at St. Michael’s College, who is a member of the ACT collaboration.

“The first image gives us a snapshot of tiny variations in the density of the primordial gas. Over millions of years, the slightly denser regions grew under the influence of gravity to form stars and galaxies. So the snapshot shows us the starting point for all of the marvelous structure we see in the universe today.

“The second image tells us the velocity of the gas and thereby reveals its dynamics. We get this map of the movement of the gas by measuring the polarization of the cosmic microwave background (CMB). We have done this to unprecedented sensitivity, giving a much clearer picture of the speed of the gas than was previously available.”

The second image gives the collaboration confidence that astrophysicists understand the behaviour of the early universe because it allows for another way of measuring how much atomic matter there is in the universe, as well as how much dark matter –  and how fast the universe is expanding. It also significantly strengthens researchers’ confidence that they understand the theory behind what’s being observed.

The new pictures of the CMB are at a higher resolution than those produced more than a decade ago by the Planck mission, a space-based telescope designed to observe the CMB. ACT measures the intensity and polarization of the light at five times the resolution of Planck and with around three times lower noise. This means the faint polarization signal is now directly visible in ACT's images.

“There have been many results over the years, but this is the most impressive in terms of data volume and the area of the sky covered,” says Richard Bond, a University Professor at U of T’s Canadian Institute for Theoretical Astrophysics (CITA) and an ACT collaboration member.

“Toronto played a big role in both the Planck mission to study the CMB and in ACT,” says Bond. “And it is that one-two punch that determined with incredible precision the standard model of cosmology. It is quite amazing.”

The new results confirm a simple model of the universe and have ruled out most competing alternatives, according to the research team. The work has yet to go through peer review, but the researchers have submitted a suite of papers to the Journal of Cosmology and Astroparticle Physics and the results were presented at the American Physical Society’s annual meeting on March 19.

The ACT collaboration includes faculty, postdoctoral researchers and students from the University of Toronto.

Yilun Guan, a postdoctoral researcher at the Dunlap Institute for Astronomy & Astrophysics, a Schmidt AI in Science Fellow, and a co-lead author of the latest research, led two mission-critical components of ACT analysis: data selection and calibration.

“These efforts were essential in producing this result, the most sensitive CMB map to date, covering over 40 per cent of the sky at high resolution – a milestone in modern observational cosmology,” he says.

Longtime members of the collaboration and co-authors include: Hincks, Bond and Renée Hložek, an associate professor in the department of astronomy and astrophysics and the Dunlap Institute for Astronomy & Astrophysics. A more recent member of the collaboration is Simran Nerval, a graduate student in the department.

“I've been involved in ACT since starting my DPhil in 2008 and these results represent the cumulative work of so many people over those many years,” says Hložek. “Also, it's a real privilege to see my student Simran leading parts of the analysis of one of the papers and generating the 'final ACT’ version of a plot I made for ACT in 2012.”

Other Canadian contributors include researchers from the University of British Columbia and McGill University. In addition, Toronto has long played a key role by providing computing resources for ACT on the Niagara supercomputer of the SciNet High Performance Computing Consortium at U of T – both to local ACT members and to members in their international collaboration.

Measuring the universe’s infancy

ACT’s new measurements have also refined estimates for the age of the universe and how fast it is growing today. The infall of matter in the early universe sent out sound waves through space, like ripples spreading out in circles on a pond.

A younger universe would have had to expand more quickly to reach its current size and the images we measure would appear to be reaching us from distances that are closer. The apparent extent of ripples in the images would be larger in that case, in the same way that a ruler held closer to your face appears larger than one held at arm’s length.  

The new data confirm that the age of the universe is 13.8 billion years, with an uncertainty of only 0.1 per cent.

The CMB and the Hubble tension

The result also provides an important measurement of the Hubble constant, the rate at which space is expanding today. Previous measurements derived from the CMB have consistently shown an expansion rate of 67 to 68 kilometers per second per megaparsec (about 3.26 million light years), meaning that a galaxy one megaparsec from Earth is receding from us at 67 to 68 kilometres per second.

In contrast, measurements derived not from the CMB but from the movement of nearby galaxies indicate a Hubble constant as high as 73 to 74 kilometres per second per megaparsec. This disagreement between the values is what astronomers refer to as the Hubble tension.

A major goal of the work was to investigate alternative models for the universe that would explain the disagreement and refine the value of the constant, including: changing the way neutrinos and dark matter behave; adding a period of accelerated expansion in the early universe; or even changing fundamental constants of nature.

Using their newly released data, the ACT team confirmed the lower value for the Hubble constant with increased precision and showed no evidence for the need for alternative models. According to the collaboration, the new result means the standard model of cosmology has passed an extraordinarily precise test.

ACT completed its observations in 2022, and attention is now turning to the new, more capable Simons Observatory at the same location as the now decommissioned ACT in Chile.

“As we look to the new observatory – which achieved first light this month and which will continue CMB observations – it really feels like the scientific circle of life, with new telescopes starting just as we release our final ACT results to the community,” Hložek says.

“I joined the ACT collaboration at the beginning of my PhD in 2021,” adds Nerval. “I have always been interested in answering the big questions surrounding our universe and working with ACT has allowed me to constrain models of the universe using the most precise maps of the CMB we have to date. I am glad to be continuing my work in CMB science with the Simons Observatory, both in contributing to the data pipeline and early universe theory constraints.”

Read more about the new images at Princeton University